Superplastic metallic alloys, such as for instance certain fine grain alloys of aluminum, magnesium, stainless steel and titanium, are relatively ductile and can undergo substantial tensile deformation in the presence of low shaping forces. Such materials are capable of being stretched and formed at suitable forming temperatures over a forming tool or into a die cavity to make complex shaped parts, e.g., automotive body parts, or the like. This process is often referred to as superplastic forming.
In superplastic forming, a sheet metal blank is positioned with one side lying close to the hot forming surface of a heated forming tool in a press. The metal sheet is often preheated to its forming temperature, and gripped at peripheral edges between complementary opposing dies. A pressurized fluid, such as air, is applied to the other side of the metal sheet, thereby forcing and stretching the metal sheet into conformance with the forming surface of one die while maintaining a target strain rate for deforming the sheet throughout the forming cycle. The opposing die provides an air chamber on the pressurized side of the metal sheet. The superplasticity of the material enables forming of complex components that normally cannot be formed by conventional room temperature metal forming processes. For instance, use of the superplastic forming process enables forming a workpiece with a deep cavity or with a cavity formed over very small radii. Further, superplastic forming often permits the manufacture of large single parts that cannot be made by other processes such as sheet metal stamping. A single part formed using superplastic forming can sometimes replace an assembly of several parts made from non-superplastic forming materials and processes.
In production operations, heated sheet metal workpieces are repeatedly placed on the press, formed on the heated tool, and removed. Sliding contact between the deforming metal workpiece and the forming tool often leads to problems associated with friction and adhesion. Typically, lubricants are used in this process to ease material flow over the forming surfaces of the forming tool. The lubricants also act as a release aid to prevent parts from sticking to the tool surfaces. Known lubricant technology includes a boron nitride, water-based slurry with a binder system to promote adhesion of the boron nitride to the metal sheet, and graphite slurries. Of course, both the boron nitride slurry and the graphite slurry have several drawbacks. First, the solid lubricant, e.g. boron nitride, tends to build up quickly in the die, resulting in maintenance down time for cleaning. If not removed regularly, this buildup can collect and harden in the die resulting in a defect on a freshly formed part. Further, the use of solid lubricants can lead to defects in the finished part, such as for instance skid lines or slip lines, as a result of the metal sheet slipping over sharp features on the forming tool.
It would be beneficial to provide a system and method for applying a coating of a lubricant material to a surface of a sheet metal blank, which overcome at least some of the above-mentioned limitations of the prior art.